Gas discharge lamps provide high efficiency illumination and relatively long life with exceptional color rendering qualities. The electrical characteristics of high intensity discharge (HID) lamps depend on an operational state of the lamp (e.g., ignition, glow to arc, run up, and steady state), chemistry of the components in the lamp, manufacturer variations, lamp to lamp variations, and electrode design. HID lamps generally have negative resistance characteristics on time-scales greater than a few milliseconds such that a ballast is required to fix an operating point to prevent thermal runaway of the lamp. High efficiency electronic ballasts operate on relatively slow time-scales and regulate average lamp power or current to prevent thermal runaway.
Significant electrode-plasma interactions take place on time-scales much shorter than the frequencies at which these electronic ballasts operate. Evaporation and sputtering from an electrode of the lamp correlate with lamp current and cause deposition of electrode material on a wall of an arc-tube of the lamp which reduces lamp life. Therefore, the relatively slow current control response time of electronic ballasts (i.e., controlling average current or power to the lamp) has an adverse effect on lamp life. This effect is exaggerated in electronic ballasts supporting features such as high-current starting (e.g., instant-light or instant-on), dimming, and hot re-strike where adverse electrode-plasma interacts are particularly likely.
Analog ballasts can be configured to operate on faster time scales for increased control over lamp current and reduced adverse electrode-plasma interactions. However, they do not have the flexibility of a digital controller (i.e., high current starting, dimming, hot re-strike and other features). Further, analog systems have limited abilities to adapt to various lamp operating states. For example, run-up and steady-state have very different dynamic impedance characteristics such that one set of analog control parameters may result in instabilities in one of the operating states. Analog control systems (i.e., analog ballasts) are also essentially linear or quasi-linear devices such that incorporating special conditional responses is far more difficult and costly than with a digital approach (i.e., electronic ballasts).